The present invention relates to an engine brake control in an automatic transmission.
Automatic transmissions effect a shift between various speeds having different gear ratios by a change-over in the power delivery path after selective actuation of one or more kinds of friction elements. It is the commonly employed practice to provide a one-way clutch which serves as a reaction member against a rotary member to establish a power delivery path.
With this one-way clutch, if the rotary member is subject to reverse torque, the one-way clutch is released to interrupt the transmission of this reverse torque. However, since the one-way clutch interrupts the transmission of reverse torque, no effective engine braking results.
Thus, a known automatic transmission is provided with a friction element which is arranged in parallel to a one-way clutch such that the friction element is activated to effect engine braking when a driver places a manual selector valve at a predetermined drive range position.
In the case of known automatic transmission of the THM-700 type manufactured by General Motors Corporation in the United States, a single friction element is commonly used in effecting engine braking during running with each of a plurality of speeds and the required capacity of this friction element for engine braking operation during running with the highest speed is considerably small as compared to the required capacity for engine braking operation during running with a speed lower than this highest speed.
In order to tailor the capacity of the engine brake friction element with the required capacity for running with each speed, it has been the conventional practice to use normal line pressure as the working hydraulic fluid pressure for activating the friction element when engine braking is to be effected during running with the highest speed and boost the line pressure when engine braking is to be effected during running with a speed other than the highest speed.
However, since the line pressure is used as the working hydraulic fluid pressure for activating friction elements other than the engine brake friction element, it has been impossible to set the line pressure at a low level which would provide the engine brake friction element with the required capacity for effecting engine braking operation during running with the highest speed. Thus, there has arisen a problem that when engine braking is to be effected during running with the highest speed by activation of the engine brake friction element with the normal line pressure, substantial shocks take place upon effecting the engine brake operation because the engine brake friction element is provided with an overabundance in capacity.
An object of the present invention is to solve the above mentioned problem encountered in the conventional engine brake control in automotive automatic transmission.
More particularly, an object of the present invention is to provide an engine brake control wherein a shift to engine brake operation is made without substantial shocks.